Poly Quad Revisited

by Mike Holler

David Vizard published his article on the Poly Quad technology and elaborated on the Mitsubishi he successfully applied it to.  I contacted him to tell him that I had done something similar a little over a year ago on an SRT-4 head, a guy in the Midwest — for a 2.2/2.5 block.  I explained the differences between what I did and what he did, and the direction I’ve gone since doing this head.  This inspired me to do an article on the whole concept.

The SRT-4 head was developed similarly to Vizard’s Poly Quad article.  I used one oversized intake valve and one stock valve.  On mine, however, the exhaust valves were the same size.  The large intake valve received a corresponding enlarged part, while the stock valve port was just smoothed over for better flow.  As a side note, I try to get my ports to flow so that they choke just over intended valve lift.  This keeps velocity high.  Any more flow than that will stagnate the ports and reduce power and economy.  The exhausts were ported in the typical style, where flow is king, but with a similar bias.

Since putting that head together, I have moved in a new direction.  All valves are the same size.  If I’m targeting high RPM high flow, then the valves are all BIG.  For street heads, I keep them stock sized.  Where the correlation to the Poly Quad comes into view is how I shape the ports and chambers.  I found a way to accomplish the PQ goals with more flow.  All 4 valves can be oversized if needed.  The secret is in the biasing of the ports.

Looking down through the intake port from the manifold side, one side port has the floor removed to provide a straight path in and across the back of the valve.  The other side has the roof re-radiused so that the flow will arch and splay across the back of the other valve.  Of course, to keep velocity high, the CSA must be kept in check.  This is accomplished by filling in the roof of the first side and the floor of the second side, then reshaping.  In the picture you can see the extreme biasing of the intake ports.  This was done on a Toyota 1.8 liter VVTi head.

Some 4-valve heads have a squish pad between the intake and exhaust valves, for a total of 4 squish pads.  These heads offer some unique opportunities to control combustion activity employing the PQ style of combustion chamber shaping.  These pictures (above) are of a Dodge 3.5 liter V-6 head.  Notice how the valve that has the straight through port has the side of the squish pad carved out, and the splayed valve has just enough material removed to unshroud the valve.

The exhaust ports can receive similar treatment, except there is no proven method of filling in the ports except for welding. 

Shy of welding the ports, CSA can be increased in the direction of bias, but must be limited so that they aren’t opened too much and velocity is lost.

The combination of port biasing and combustion chamber shaping yields a very active swirl effect that better vaporizes and homogenizes the air/fuel charge.  Power is up due to more of the chemical energy being converted into kinetic energy sooner in the power stroke.  Detonation is down due to increased chamber activity and a better homogenized charge.

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